IgnoreUnknown UseKeychain  # because macOS https://developer.apple.com/library/content/technotes/tn2449/_index.html

Host * # Defaults for all hosts defined after this
  AddKeysToAgent yes
  UseKeychain yes
  UseRoaming no
  ForwardAgent no
  IdentitiesOnly yes
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/id_ed25519_profile_a

Host Example
  HostName example.com

Host Sample
  HostName sample.org
  IdentityFile ~/.ssh/id_ed25519_profile_b